Considerable resources are spent by government, university and industrial agencies in identifying and counting of commercially important fish and other aquatic organisms. Manual collection of such information consumes considerable time, is subject to human error, and tends to cause changes in the behaviour of the organisms. Accordingly, it would be desirable if the identifying and counting of aquatic organisms could be achieved without constant human monitoring.
A number of devices are known for the detecting and counting of fish and other aquatic organisms. Examples of prior devices are disclosed in U.S. Pat. Nos. 3,040,980, 4,743,742 and Canadian patent No. 1,070,707. A device that includes the use of a TV camera is disclosed by M. H. Beach - The use of infra-red and closed circuit TV to validate records form automatic fish counters - J. Fish Biol. (1978).
Machine vision is most efficient when the image of interest is presented as a silhouette with a uniform brightly lit background. Also, with a machine vision system using a perimeter tracing algorithm, it is desirable that the entire target remain within a brightly lit background.
The conventional approach for providing a silhouette image has the lighting behind the object. The known systems used for monitoring aquatic organisms are not entirely satisfactory in providing high quality images under a wide variety of changing environmental conditions, including variation of light intensity for night and day, water current flow an depth, and water color and turbidity. Also, the known systems are inefficient in the utilization of energy for producing illumination, which is particularly significant to operating in remote locations.